﻿using System;
using System.Collections.Generic;
using System.Text;

namespace ST.Common.Data
{
	/// <summary>
	/// From: https://github.com/twitter/snowflake
	/// An object that generates IDs.
	/// This is broken into a separate class in case
	/// we ever want to support multiple worker threads
	/// per process
	/// </summary>
	public class SnowFlake
	{
		private long workerId;
		private long datacenterId;
		private long sequence = 0L;

		private static long twepoch = 1288834974657L;

		private static long workerIdBits = 5L;
		private static long datacenterIdBits = 5L;
		private static long maxWorkerId = -1L ^ (-1L << (int)workerIdBits);
		private static long maxDatacenterId = -1L ^ (-1L << (int)datacenterIdBits);
		private static long sequenceBits = 12L;

		private long workerIdShift = sequenceBits;
		private long datacenterIdShift = sequenceBits + workerIdBits;
		private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
		private long sequenceMask = -1L ^ (-1L << (int)sequenceBits);

		private long lastTimestamp = -1L;
		private static object syncRoot = new object();

		public SnowFlake(long workerId, long datacenterId)
		{

			// sanity check for workerId
			if (workerId > maxWorkerId || workerId < 0)
			{
				throw new ArgumentException(string.Format("worker Id can't be greater than %d or less than 0", maxWorkerId));
			}
			if (datacenterId > maxDatacenterId || datacenterId < 0)
			{
				throw new ArgumentException(string.Format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
			}
			this.workerId = workerId;
			this.datacenterId = datacenterId;
		}

		public long NextID()
		{
			lock (syncRoot)
			{
				long timestamp = TimeGen();

				if (timestamp < lastTimestamp)
				{
					throw new ApplicationException(string.Format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
				}

				if (lastTimestamp == timestamp)
				{
					sequence = (sequence + 1) & sequenceMask;
					if (sequence == 0)
					{
						timestamp = TilNextMillis(lastTimestamp);
					}
				}
				else
				{
					sequence = 0L;
				}

				lastTimestamp = timestamp;

				return ((timestamp - twepoch) << (int)timestampLeftShift) | (datacenterId << (int)datacenterIdShift) | (workerId << (int)workerIdShift) | sequence;
			}
		}

		protected long TilNextMillis(long lastTimestamp)
		{
			long timestamp = TimeGen();
			while (timestamp <= lastTimestamp)
			{
				timestamp = TimeGen();
			}
			return timestamp;
		}

		protected long TimeGen()
		{
			return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
		}
	}
}
